Electronic devices and their components, including solder points and other interfaces, packaging materials, and printed circuit board(s), are advantageously designed to withstand at least a minimum amount of wear and tear. To determine and subsequently improve the reliability, researchers usually perform a battery of component level testing, including temperature testing, stress testing, and moisture testing.
From the mechanistic standpoint, reliability can be measured by investigating a combination of elastic, plastic, and viscoelastic behaviors of materials. For example, solder ball/underfill interface failure under may originate from a combination of plastic deformation of the solder ball and viscoelastic flow of the underfill. From that perspective reliability can be thought of as a multiple interfacial interaction with, for example, the solder-solder, polymer-polymer, polymer-solder and the associated stress/strain relationships contributing to the predicted failure.
However, for the chemist looking at polymer-involved interfaces, the determination of failure is not that simple. Failure can occur both on a relatively large level, for example from the multiple interfaces, as defined by the engineer, and also on a much smaller level from specific contributions at the atomic and molecular level. To the chemist it is the investigation of the structure at the molecular and even atomic levels that will lead to solutions of the problem. The basic concern then becomes determining the atomic and molecular causes of the failure, especially if the chemist must correct the mechanism.
From the chemist's perspective, polymer performance relies on a combination of bond-related and non-bond-related energy contributions. For instance microstructural domains, which are often studied to understand the link between morphology and engineering performance, represent a macro-scale manifestation of the energy balance originating from the molecular structure. That is, such features originate from the way in which the specific molecular structure responds to the chain structure and its relative orientation with neighboring surfaces. Orientation is also a key parameter that decides a polymer interaction, especially when looking at substrate effects in which interfacial orientation creates properties different than the bulk. So for the polymer chemist looking for the failure mechanism, several questions are always considered: a) whether the interchain interactions low enough so that only bond forces are important to the mechanical property; b) whether the through space interactions which impact orientational effects more important; or c) whether the balance of bond and through-space responses the most critical consideration to understand.
Consider the mechanism of a cycling experiment. Failure can be established by following the same mechanism as in a pure tensile or shear test. Failure can also be established and studied on the molecular level, since relative chain orientations will be constantly changing during each cycle. This change of relative chain orientations suggests that the energy drivers, which set up the orientations and the domains within that bond line, help to determine the bond strength, frequency responses and the ultimate failure. In addition, the adhesive failure depends upon the population of interfacial interactions at the surface. Failure, especially for cycling, then becomes understanding the shifting nature of the interactions that is governed by how the polymer responds to the specific stress. Network structures are even more complicated, supposedly infinite in dimension. However, given the example above, and the geometric and diffusional limits imposed on creating that infinite universe, a simple assumption can be drawn that very few highly chemically crosslinked networks actually are formed that reach from top to bottom through the bond line. It is then how the various networks interact that will form the basis of the performance of the material, whether defined as interpenetrating or entangled. The relative populations of orientations and their interactions will then help determine the survivability of the interface.
For the formulation chemist and the computational chemist then, it is the smaller universe that is addressed for performance issues. The limited assumption taken in the current studies is to understand the potential interactions of the polymer backbone. The basic drivers for structural performance to the organic chemist has always been, higher polarity, higher hydrogen bond characteristic and higher rigidity leads to higher strength. In reality, a tradeoff exists between strength, toughness, and modulus. To the computational chemist it is also clear that these tradeoffs consist of a structural balance between the bond rotational and vibrational movements, and the through space interactions or attractions that constrain local translation. Simple addition of a polar group does not always lead to a more reliable, or “stronger” interface. For the chemist, a simple correlation to structure is sought, leading to the current investigation of reliability issues on a molecular scale.
Therefore, there is still a need to reliably and repeatedly determine the likelihood and degree of failure of particular known combinations of polymers and substrates that can form an interface without excessive or undue “real-time” experimentation by the researcher. There is also a need to model and preferably predict the success and failure rates of particular combinations of novel or known classes of polymers and types of substrates that can be used to form an interface, in order to minimize futile research efforts and to minimize the costs of real-time experimentation.